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Chart tooltips using quadtrees

Adding tooltip interactivity to charts is one of the most desired features, but also one of the tricker ones to implement. One common solution is to attach .on('mouseover') event handlers to each circle element. This will work okay, except when points start overlapping. You will also start using up lots of memory if the number of points is very large.

A more advanced technique is to use d3.geom.voronoi. This solves the problem of overlapping points, but you still run into performance issues. That's because

	#!/usr/bin/env python
	# -- coding: utf-8 --
	#
	# Copyright (C) 2011 Radim Rehurek <[email protected]>
	# Licensed under the GNU LGPL v2.1 - http://www.gnu.org/licenses/lgpl.html
	#
	# Parts of the LDA inference code come from Dr. Hoffman's `onlineldavb.py` script,
	# (C) 2010 Matthew D. Hoffman, GNU GPL 3.0

	function eachKey(obj, f) {
	for(var key in obj) {
	if( obj.hasOwnProperty(key) )
	f(key, obj[key]);
	}
	}

	function adtcase (base, proto, key) {
	return (...args) => {
	var inst = new base();

	var AmpersandCollection = require('ampersand-collection'),
	_ = require('underscore'),
	AmpersandRestCollection = _.clone(require('ampersand-collection-rest-mixin')),
	sync = require('./sync'),
	Search = require('./search'),
	EJSON = require('mongodb-extended-json'),
	AmpersandModel = require('ampersand-model');

	var MongoDBCollection = AmpersandCollection.extend(AmpersandRestCollection, {
	fetched: false,

	/*
	The MIT License (MIT)

	Copyright (c) 2014

	Permission is hereby granted, free of charge, to any person obtaining a copy
	of this software and associated documentation files (the "Software"), to deal
	in the Software without restriction, including without limitation the rights
	to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
	copies of the Software, and to permit persons to whom the Software is

	// Use this helper function in the model's constructor or initialize function to link a state property to
	// a parent collection, using `linkIdKey` to look up the child state.
	// Very handy for keeping a sticky reference to a single item in a collection and still maintain
	// all the goodness of a child state (like change event bubbling).
	// Derived properties that use the linked state property will work as expected.
	//
	// NOTE: DO NOT use a derived property to accomplish this. While it may seem tempting at first,
	// ampersand collections do not emit `change` events when the collection itself or any of the
	// children change (this would be very inefficient). Because of this, the derived property would have
	// no way to know when to update. This helper fixes that by tracking changes to the collection

	// This gist is deprecated, doesn't work with newer versions of JSDom.
	// Instead, use https://www.npmjs.com/package/highcharts-jsdom